Dear all,
I'm new to UMATHT subroutine and have recently started learning the general aspects of it. I've been trying to validate the results obtained from the user subroutine with the results obtained using normal material properties in ABAQUS interface. To do so, I prepared a series of simple simulations which consisted of isothermal heat transfer, non-isothermal (linear dependency) and non-isothermal (non-linear dependency). I used a simple surface heat flux as input and used fixed increments so that I could compare the results in the same increments for each variable.
In the isothermal case, the results of the user subroutine and the normal material properties are identical for all of the variables in every increment.
As for the non-isothermal case, if thermal conductivity is kept as a constant and specific heat is assumed temperature dependent, again the results match perfectly for every variable. But if the thermal conductivity is assumed to be temperature dependent, the results are no longer identical(Specially the HFL variable provided by ABAQUS). This goes for both linear and non-linear dependency. I haven't found a reference which has validated the heat flux vector in both cases for the temperature dependent case and I've compared my general coding with other UMATHT subroutines provided by others and haven't found anything different and was wondering if you could help me find what I'm doing wrong. Thank you in advance for helping.
Here's the subroutine I'm using for the fixed specific heat and temperature dependent thermal conductivity case. I'm going to attach the CAE file at the end if you would like to see the model as well.
SUBROUTINE UMATHT(U,DUDT,DUDG,FLUX,DFDT,DFDG,
1 STATEV,TEMP,DTEMP,DTEMDX,TIME,DTIME,PREDEF,DPRED,
2 CMNAME,NTGRD,NSTATV,PROPS,NPROPS,COORDS,PNEWDT,
3 NOEL,NPT,LAYER,KSPT,KSTEP,KINC)
C
INCLUDE 'ABA_PARAM.INC'
C
CHARACTER*80 CMNAME
DIMENSION DUDG(NTGRD),FLUX(NTGRD),DFDT(NTGRD),
1 DFDG(NTGRD,NTGRD),STATEV(NSTATV),DTEMDX(NTGRD),
2 TIME(2),PREDEF(1),DPRED(1),PROPS(NPROPS),COORDS(3),
3 DFLUX(NTGRD)
REAL*8 TEMPDT,Specht,Cond,K0,K1,T0,T1,dt,dk,DCond
TEMPDT=TEMP+DTEMP
Specht=PROPS(1) !specific heat (constant)
K0=PROPS(2) !thermal conductivity at Temp0
K1=PROPS(3) !thermal conductivity at Temp1
T0=PROPS(4) !Temp0
T1=PROPS(5) !Temp1
if (TEMPDT.GT.T1) then
Cond=K1
DCond=0.d0
elseif (TEMPDT.LT.T0) then
Cond=K0
DCond=0.d0
elseif (TEMPDT.GE.T0.AND.TEMPDT.LE.T1) then
dk=K1-K0
dt=T1-T0
DCond=dk/dt
Cond=DCond*(TEMPDT-T0)+K0
endif
! Internal Energy Calculation
DUDT=Specht
U=U+DUDT*DTEMP
! Heat Flux Calculation
DO I=1,NTGRD
FLUX(I)=-Cond*DTEMDX(I)
DFDG(I,I)=-Cond
DFDT(I)=-DCond*DTEMDX(I)
ENDDO
RETURN
END
I'm new to UMATHT subroutine and have recently started learning the general aspects of it. I've been trying to validate the results obtained from the user subroutine with the results obtained using normal material properties in ABAQUS interface. To do so, I prepared a series of simple simulations which consisted of isothermal heat transfer, non-isothermal (linear dependency) and non-isothermal (non-linear dependency). I used a simple surface heat flux as input and used fixed increments so that I could compare the results in the same increments for each variable.
In the isothermal case, the results of the user subroutine and the normal material properties are identical for all of the variables in every increment.
As for the non-isothermal case, if thermal conductivity is kept as a constant and specific heat is assumed temperature dependent, again the results match perfectly for every variable. But if the thermal conductivity is assumed to be temperature dependent, the results are no longer identical(Specially the HFL variable provided by ABAQUS). This goes for both linear and non-linear dependency. I haven't found a reference which has validated the heat flux vector in both cases for the temperature dependent case and I've compared my general coding with other UMATHT subroutines provided by others and haven't found anything different and was wondering if you could help me find what I'm doing wrong. Thank you in advance for helping.
Here's the subroutine I'm using for the fixed specific heat and temperature dependent thermal conductivity case. I'm going to attach the CAE file at the end if you would like to see the model as well.
SUBROUTINE UMATHT(U,DUDT,DUDG,FLUX,DFDT,DFDG,
1 STATEV,TEMP,DTEMP,DTEMDX,TIME,DTIME,PREDEF,DPRED,
2 CMNAME,NTGRD,NSTATV,PROPS,NPROPS,COORDS,PNEWDT,
3 NOEL,NPT,LAYER,KSPT,KSTEP,KINC)
C
INCLUDE 'ABA_PARAM.INC'
C
CHARACTER*80 CMNAME
DIMENSION DUDG(NTGRD),FLUX(NTGRD),DFDT(NTGRD),
1 DFDG(NTGRD,NTGRD),STATEV(NSTATV),DTEMDX(NTGRD),
2 TIME(2),PREDEF(1),DPRED(1),PROPS(NPROPS),COORDS(3),
3 DFLUX(NTGRD)
REAL*8 TEMPDT,Specht,Cond,K0,K1,T0,T1,dt,dk,DCond
TEMPDT=TEMP+DTEMP
Specht=PROPS(1) !specific heat (constant)
K0=PROPS(2) !thermal conductivity at Temp0
K1=PROPS(3) !thermal conductivity at Temp1
T0=PROPS(4) !Temp0
T1=PROPS(5) !Temp1
if (TEMPDT.GT.T1) then
Cond=K1
DCond=0.d0
elseif (TEMPDT.LT.T0) then
Cond=K0
DCond=0.d0
elseif (TEMPDT.GE.T0.AND.TEMPDT.LE.T1) then
dk=K1-K0
dt=T1-T0
DCond=dk/dt
Cond=DCond*(TEMPDT-T0)+K0
endif
! Internal Energy Calculation
DUDT=Specht
U=U+DUDT*DTEMP
! Heat Flux Calculation
DO I=1,NTGRD
FLUX(I)=-Cond*DTEMDX(I)
DFDG(I,I)=-Cond
DFDT(I)=-DCond*DTEMDX(I)
ENDDO
RETURN
END